1. Field of the Invention
The system of this invention pertains to an improved apparatus and method for bonding beam lead devices and flip chip devices onto mating conductive surfaces on a substrate or other surface and for bonding electrical leads onto semiconductor devices and subsequently bonding said leads onto mating conductive surfaces on a substrate or other surface, utilizing either ultrasonic or thermocompression energy. The bonder of this invention utilizes a novel bonding tool having minute spherically shaped bonding surfaces which are caused to individually and successively contact and bond each of a plurality of electrical leads projecting from the periphery of a beam leaded device onto a substrate. A novel pivoting mechanism is disclosed for causing a complex wobbling motion of the bonding tool so that the tool follows an adjustable rectangular or other predetermined linear path around the periphery of the device to produce the desired individual and successive bonding of the leads. In the ultrasonic configuration, a novel frequency-modulated ultrasonic generator insures that the correct amount of ultrasonic energy is applied to each bond and that the system is always in resonance. A novel composite will also be disclosed in a related application which enables the system of this invention to bond electrical leads to a semiconductor device as well as bond them to a substrate or other surface having conductive patterns.
2. Description of the Prior Art
There are three presently used techinques of beam lead bonding. One is called a tacking technique which can utilize either thermocompression or ultrasonic energy. In this technique one lead at a time is put down and bonded with a tacking probe.
The second technique is called wobble bonding. There are presently three known variations of wobble bonding. One utilizes a wobble bonder such as that disclosed in U.S. Pat. No. 3,575,333. This patent discloses a thermocompression bonding tool mounted in a housing and pivoted about a focal point at the working surface to engage sequentially the electrical leads. The wobbling movement is generally in a conical or eliptical fashion and the bonding tool generally has a flat bonding surface but sometimes uses a slightly rounded tool. However, in practice, this type of wobble bonder is limited to thermocompression energy. It uses heat on the tool and heat on the work station. The pivot point of this tool is in axial alignment with the longitudinal axis of the bonding tool and in practice cannot be adapted to ultrasonic bonding.
A second variation of wobble bonding is disclosed in U.S. Pat. No. 3,700,156 in which an ultrasonic wobble bonder is disclosed. In this wobble bonder, the tool holder which transmits ultrasonic energy is pivotally mounted in radial fashion, that is, the longitudinal axis of the tool is perpendicular to the longitudinal axis of the tool holder. In this system the ultrasonic transducer as well as the bonding tool is wobbled.
A third variation of wobble bonding is achieved by wobbling the work table rather than the bonding tool. Within the wobble table systems there are two variations. In one the table is wobbled to bring each lead into sequential contact with the bonding tool, as disclosed in U.S. Pat. No. 3,672,034. In the other, the table is wobbled to a position such that the beam leads are planar relative to the bonding tool. The tool is then lowered and all leads are bonded simultaneously. A recent patent disclosing this technique is U.S. Pat. No. 3,475,814. This variation is not a true wobble bonding system, but is mentioned here because some wobbling of the table is necessary to achieve the desired result.
The third principal technique of beam lead bonding is compliant bonding. In compliant bonding, the bonding tool is kept planar relative to the substrate. A compliant material, such as soft aluminum, is placed between the tool and the beam leads. The purpose of this material is to equalize the pressure on the leads since they are usually all bonded at one time.
Each of the bonding systems and techniques disclosed in the prior art has its own advantages and disadvantages. The bonding system of the present invention is a complex machine utilizing a variety of novel features which both overcome the disadvantages of the units disclosed in the prior art and combine their many advantages in one system. The system of the present invention is a significant improvement over U.S. Pat. No. 3,700,156 owned by the same assignee.
The system of the present invention is the first commercially available wobble bonder of the wobbling tool variety in which the wobble movement does not describe merely a conical or eliptical path around the periphery of a beam lead or other semi-conductor device. Whether a beam lead or other device is square or rectangular in shape, prior wobble bonders trace a conical or an eliptical path around its periphery and thus bond some leads closer to the device than others or bond with varying pressure or direction of force. This can result in an uneven bonding pattern, undesirable electrical characteristics in the circuit and an occasional missed bond. The system of the present invention, however, enables the bonding tool to wobble in a straight line along the x-axis and the y-axis a predetermined distance from the edge perimeters and parallel to the edge perimeters of the device, or in any other desired linear path, and even on the surface of the device. Hence, the tool of the present invention does not wobble in the traditional sense. Its motion might better be described as a rocking motion and the bonder should be termed a rocker bonder.
There are three novel factors which contribute to this unique wobble or rocker motion and which distinguish the system of the present invention from the prior art. First, this system utilizes the design disclosed in U.S. Pat. No. 3,700,156 in which the longitudinal axis of the bonding tool is perpendicular to the longitudinal axis of the tool holder. This is the only design known in the present art which permits the use of ultrasonic energy in a bonder having a wobbling tool. However, in the present invention, the wobbling mechanism is improved to permit the tool to follow a rectangular rather than an eliptical path. The second novel factor contributing to the rocker bonder is the bonding tool itself. The prior art discloses in U.S. Pat. No. 3,505,726 a rounded bonding tool which minimizes the lateral forces applied to a beam when rocking over the leads. However, the design of the tool is such that it can bond only one of four sides at a time and then must be re-set. The bonding surfaces on the tip of the bonding tool of the present invention are four minute sections of a sphere disposed about the periphery of the tool which are utilized sequentially in wobbling or rocking around the periphery of a beam lead device. Other bonding tools of more conventional designs may be utilized with the system of the present invention, but this novel design yields significantly improved results. The third novel factor is that the speed of the bonding tool over a given direction can be varied so that each lead receives the same amount of bonding energy. This speed can be a function of the number of leads to be bonded along a perimeter of the device. Thus the present invention marks a significant improvement over the prior art.
In addition to the above points of novelty, the wobble bonding system of the present invention permits the bonding of lead composites to semiconductor devices as well as to a substrate or other surface having conductive patterns. This is made possible by the novel rectilinear motion of the bonding tool, the precise control of tip pressure on the leads and the precise control of the bonding energy.
To bond the lead composites both to the device and to a substrate or surface requires two trips of the tool around the periphery of the device, bonding the leads to the device on the first and the leads to the substrate on the second.
The wobble bonding system of the present invention also includes a frequency modulated ultrasonic generator incorporating an added saw tooth sweep or other function generator wherein the center frequency is modulated by the use of a saw tooth sweep generator; and the saw tooth sweep generator combined with adjustable speeds for wobbling the bonding tool around the perimeter of the device, permits the system to achieve more uniform bonds over the entire range of leads through an even distribution and control of the ultrasonic energy.
In addition to combining the aforesaid advantages of prior machines, the bonding system of the present invention may be utilized as a compliant bonder and as a flip chip bonder by reducing the pivotal movement of the bonding tool on its x- and y-axes to 0, 0.
The bonding system of the present invention also permits the bonding of a variety of metals directly to a semiconductor chip either ultrasonically or by thermocompression through the utilization of novel beam and lead composites.